Ok, so when you look at a report, you can find a full break down of the factors that affect your fighting strength by hovering over the percentage:
Let's work left to right:
Faith: This is determined by your presence in the province you are fighting in. No Church=50%, Church1=100%, Church2=105%, Church3=110% (offensively)
Morale: This is determined by your size compared to the player, early on, once you pass a mark of about 4 times the size of your target your morale will start to drop. I believe Morale can pull you down to 35% if you seriously outweigh your target, if you attack this kind of target without faith it drops to 17%, about the worst it can get.
Misfortune/Luck: +/- as much as 15% randomly assigned to the battle to the perspective of the attacker. If you have a Misfortune of 8% then your fighting strength drops by as much.
Weapon Mastery: 5 levels through tribe skills, increments 2% per level.
Officer: Crown bought or earned Officer card that gives you an additional 10% attack strength when used.
If you have church, it is likely you are attacking a considerably smaller target than yourself.